The operation of a dual mode (analog and digital) cellular mobile telephone system is set forth in an Electronic Industry Association/Telecommunications Industry Association EIA/TIA Interim Standard entitled "Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard", IS-54-B (April 1992). The dual-mode EIA standard IS-54-B has features found in the conventional analog Advanced Mobile Phone Service AMPS system and, in addition, digital voice transmission features.
A flexible increase of capacity is provided by dividing existing 30 kHz bandwidth analog channels into a plurality of digital Time Division-Multiple Access (TDMA) channels. As a result, the same cell area can support both analog and digital transmission of voice and control or signalling information. The preferred implementation of the mobile telephone or terminal is a dual operation apparatus which can use analog channels where there is no digital support. In addition, the IS-54-B Interim Standard requires that the mobile terminal gain access to the digital channel through the analog channel. As a result, a dual operation terminal requires all of the functionality of a conventional analog mobile terminal, while also requiring digital functionality for TDMA digital mode operation.
A preferable solution is to integrate these functions as much as possible, and to provide dual-use elements where practical to reduce complexity, weight, power consumption, and cost.
One significant difference between the analog and digital modes of operation is in the required modulation for the transmission of voice and signaling information to a base station that serves a cell within which the terminal is currently located. The conventional analog terminal employs a frequency modulation (FM) of a radio frequency (RF) carrier signal. However, the digital terminal requires the use of a .pi./4-shift differential quadrature phase shift keying (DQPSK) modulation technique. These two modulation techniques are generally incompatible with one another.
One approach to providing a dual-use modulator is disclosed in commonly assigned U.S. Pat. No. 5,124,672 (E. J. Kuisma). In this approach, analog or digital signals are applied to in-phase (I) and quadrature (Q) generators to form I,Q waveshapes which modulate an intermediate frequency and, upon summation, modulate a transmission frequency.
Another problem that is encountered when providing a dual-mode mobile terminal relates to the receiver circuitry. In general, when receiving an analog FM modulated transmission a second IF frequency of several hundred kHz is employed, in conjunction with an FM discriminator. However, when receiving a digital .pi./4 shift DQPSK modulated transmission a second IF frequency of several MHz is employed, in conjunction with a suitable DQPSK demodulator. As a result of these significantly different receiver and demodulator requirements, it is difficult to reduce receiver circuit complexity, redundancy and power consumption.